When a porous substrate needs to be treated by etching or modification of a surface thereof, damage of the substrate often occurs. This is particularly true with plasma mediated treatments. This is presumably caused by active plasma radicals penetrating deeply into the porous substrate and reacting therewith, thereby changing its composition. Both oxidative and reductive plasma have such detrimental effects. This problem is well known to occur in the field of microelectronics during integration of low-k dielectrics.
Low-k dielectrics (materials having a dielectric constant lower than the dielectric constant of SiO2, i.e. lower than 4.0) are necessary to decrease capacitance between nearby conductive portions of high density integrated circuits and thereby avoiding loss of speed and cross-talk. In order to decrease the dielectric constant of low-k dielectrics as much as possible, low-k dielectrics are made porous. Thereby, the dielectric constant can be lowered down to about 2.0 or even less. Integrated circuit fabrication processes on dielectrics involve plasma etching and expose therefore the dielectrics to the damages mentioned above. US2005/0148202 describes a method for sealing or protecting porous materials used in semiconductor fabrication. It describes sealing the pores of porous material by applying a mixture of a polymer compound and an organic solvent. The sealing layer thus formed is further dried resulting in evaporation of organic solvents and volatile constituent (if any), and securing of the polymer compound on the surface as a sealing material. Such a sealing method has however several drawbacks. The solvent used being in its liquid form when in contact with the substrate, and this for a large amount of time at a relatively high temperature, dissolution or damage of the porous layer is possible. Furthermore, the liquid solvent is susceptible to penetrate deeply in the porous material and part of it could remain entrapped below the dried polymer material, thereby changing the dielectric constant of the dielectrics.
Furthermore, the method is rather complicated, tedious and labour-intensive since it involves synthesising a particular polymer having well defined end-groups, preparing a particular polymer solution, applying it homogeneously on the substrate (this implies good wettability and elaborated application techniques), and heating the solution to evaporate solvent and/or dry the polymer. Furthermore, polymer deposition typically generates stresses in porous substrates. Also, the polymer being retained in the pores in the final product, it potentially influences the properties of the dielectrics making them harder to control. It also raises the question of mechanical stability when there is a mismatch between the thermal coefficient expansion of the polymer and of the porous material.
Dubois et al. (Adv. Mater. 2011, 23, 25, 2828-2832) discloses a similar method for sealing porous low-k dielectrics with an organic polymer. The polymer is degraded by thermal treatment once the etching and other processing steps are performed. This presumably permits the patterned dielectric in the final structure to have comparable properties to its pristine equivalent. However, removing a polymer by thermal means implies degrading it and thereby the possibility of leaving polymer residues in the pores. Also, it is energy intensive. Furthermore, the other drawbacks proper to the use of polymers remain as mentioned above for US2005/0148202.
There is therefore a need in the art for a way to prevent damages to porous substrates upon plasma treatment, which avoids the above drawbacks.